Not new at all. III Corps HQs at Ft Hood has had a Canadian Brigadier General as a Deputy Commander for years. It works very well.
Given USAREUR's role as a force provider for possible NATO missions in Europe, this is not an unusual decision at all. We fight as coalitions today, and as you can see from that German BG's bio, he has served as a part of ISAF in Afghanistan -a coalition force.
I don't see any problem with this at all, and I doubt anybody at USAREUR does either.